The invention relates to an I.F. filter arrangement for FM receiving signals.
An I.F. filter arrangement having a narrow band I.F. filter with varying centre frequencies is known from WO 88/08223, equivalent to EP 0 358 649.
The I.F. filter is disposed between the tuner and the demodulator and its centre frequency can be tracked dependent on the demodulated FM signal by means of capacitative diodes.
The desired advantage which this device should offer as opposed to conventional fixed filters is, on the one hand, a significantly increased selection with respect to neighbouring channel interferences and, on the other hand, a measurable improvement of the signal-to-noise ratio as a result of the considerable reduced bandwidth and the deviation tracking therefore applied.
These improvements are only seen in the statistical operation at sufficient field strengths and low deviation.
At high deviation, the Klirr factor in the demodulated FM signal rises very intensely up to unacceptable values.
A further disadvantage of this arrangement is seen when impulse-like disturbances or quick modulation shifts occur. This I.F. filter arrangement can then get out of control so much so that dramatic impulse disturbances become noticeable. In audio receivers having such a circuit, these disturbances are recognized as aggressive scratchings.